In general, the laying of pipelines on the bed of a body of water is carried out using laying vessels, which are equipped with production lines for constructing the pipelines and laying devices for laying the pipelines on the bed of a body of water. Construction and laying of the pipeline are carried out according to two chief methods, each of which has respective geometrical characteristics that make it preferable to the other depending on the depth of the seabed.
The first method, preferable for deep-water laying, contemplates making a pipeline using a substantially vertical production line and laying the underwater pipeline in a substantially vertical position. Pipelines laid with the first method assume a J-shape in the section extending between the laying vessel and the bed of the body of water, and the method is referred to as J-laying.
The second method, preferable for laying in shallow to medium depth waters, contemplates making the underwater pipeline using a substantially horizontal production line on board the laying vessel and guiding the underwater pipeline along a curved path defined by a supporting structure that extends downstream of the production line and partially into the body of water. Underwater pipelines laid according to the second method assume an S-shape between the laying vessel and the bed of the body of water, and the method is referred to as S-laying. In practice, the laying vessel comprises a floating structure, such as a hull or multihull for example, which houses the production line, and a supporting structure, for example a launch ramp, hinged to the floating structure.
One problem related to laying pipelines on the bed of a body of water consists in the high stresses acting on the pipeline in the launch phase. This problem is intensified by the current trends of laying pipelines of ever increasing size, and at deeper depths. In the case of S-laying, it is technically possible to keep the values of stress on the pipeline under control for the same pulling tension of the tensioners (i.e., the devices assigned to braking and controlling the release of the pipeline from the laying vessel), by increasing the exit angle of the pipeline via rigid supporting structures. However, this technical solution causes high stress on the supporting structure and the floating structure of the laying vessel and therefore requires much larger and stronger supporting structures and, at the same time, large floating structures, with a consequent significant increase in costs.
In the field of underwater pipeline laying, it is known to use support devices for supporting the outboard pipeline. These support devices have the advantage of partially offloading particularly heavy stress on the supporting structure and the floating structure, but induce concentrated shear stress on the pipeline.
In greater detail, U.S. Pat. No. 3,559,413 describes a supporting device comprising floating bodies arranged along the surface of the body of water, and saddles connected to the floating bodies to sustain the span of pipeline.
U.S. Pat. No. 5,575,590 describes a supporting device similar to the previous one, where the saddle is replaced by a clamp provided with a mechanism fitted with rollers that press on the pipeline with pistons operated by the hydrostatic pressure of the body of water. The saddle is connected to a floating body or to a floating pontoon positioned on the surface of the body of water.
U.S. Pat. No. 3,517,519 describes a supporting structure provided with floating bodies immersed in the body of water that enable reducing the stress transmitted from the supporting structure to the laying vessel.
PCT Patent Application No. WO 2013/186758, in the name of the same applicant, describes a laying system for shallow waters comprising a short ramp hinged to a laying vessel; and a laying device equipped with floating bodies immersible in the body of water and connected to a trolley movable along the part of the pipeline extending between the short ramp and the bed of the body of water.
Other types of supporting devices are disclosed in U.S. Pat. Nos. 4,127,006 and 5,576,590.
In the above-indicated documents regarding certain of the known art, the vertical buoyancy force provided by the floating bodies contributes to reducing the stress transmitted from the pipeline to the supporting structure and from the supporting structure to the laying vessel, but progressively loses efficiency as the inclination of the downward spanning portion of the pipeline grows.